All entities that consume large amounts of gasoline, diesel, and related fuels have been greatly impacted by a significant increase in cost. Furthermore, due to worldwide unrest and increased worldwide demand, inflationary costs will continue to drive up the cost of energy. To combat these ever increasing costs, many entities are adopting new technologies such as hybrid propulsion systems to lower or control these dramatic cost increases.
1. Hydraulic Hybrid Transmissions:
A hydraulic hybrid power train system typically includes: a power plant generating a high pressure fluid at an output; at least one drive motor responsive to the high pressure fluid for generating rotary motion at an output; and a mode selection device connected to the power plant output and the drive motor. The mode selection device selects a mode of operation from a plurality of drive motor modes of operation, including at least two of a drive mode, a neutral mode, a reverse mode and a park mode. The system includes a control device connected to the power plant and the drive motor for controlling operation of the drive motor in the plurality of modes of operation. The system further includes a selectively actuated brake device for interrupting high pressure fluid flow to the drive motor, and a check valve bridge circuit for connecting the drive motor to a low pressure fluid source when the brake device is actuated.
For the current disclosure, one known aspect of the design of note is the park mode. As currently practiced for hydraulic hybrid transmissions, park mode changes the position of one or more hydraulic valves to prevent oil flow from conduits that lead to the one or more drive motors. In this manner, oil is prevented from reaching the motors, thereby limiting their ability to rotate.
Another known aspect of the design is the configuration of the drive-line components of the system. As currently practiced, the front and rear drive components of hydraulic hybrid transmission equipped vehicles are mechanically independent of one another.
2. Transmission Brakes in the Present State of the Art:
Many modern vehicles with automatic transmissions provide two independent means for preventing the vehicle from moving when it is parked. The independent means include: a transmission brake and a parking brake (also sometimes referred to as an emergency brake). The former is used to prevent the transmission, and therefore the vehicle's driven wheels, from rotating. The latter is typically only applied to the vehicle's rear wheels. As such, having both of these devices provides an additional degree of safety since each, individually, is subject to a variety of failure modes. For example, in certain environments, parking brakes can fail without warning (for example, due to debris collecting between mating surfaces), causing the vehicle to move unexpectedly. Thus, having a transmission brake in addition to the parking brake can militate against accidents from occurring.
Compared to the complexity of an automatic transmission, the means for implementing a transmission brake is quite straight-forward. As shown in FIG. 1, the transmission brake mechanism 100 is implemented with a series of notches 102 on the outside of a clutch housing 104 and a single or pair of spring-loaded catches 106. To engage the transmission brake mechanism 100, an actuator 108 simply moves the catch 106 radially inwardly into one of the notches 102 to prevent the clutch housing from rotating.
Vehicles with hybrid hydraulic power trains are similar to traditional vehicles in that it is necessary to provide some means for ensuring that the vehicle stays stationary when parked. However, the means for providing a transmission lock, such as that shown in FIG. 1, cannot be applied because the transmission does not have a clutch housing.
One approach for providing a transmission braking capability is to essentially “do nothing”. Knowing that hydraulic fluid is essentially incompressible and that the hydraulic pump is coupled to the engine, when the vehicle's engine is turned off, the amount of torque that would be required to back-drive the system should be sufficiently great that the vehicle should not move. There are two flaws with this approach, however. First, many hydraulic hybrid vehicles are designed to capture braking energy, which is essentially back-driving the system. As such, the amount of torque to back-drive the system is not significantly large. In addition, some hydraulic hybrid vehicles use the hydraulic motor to start the engine. Thus, if the vehicle were to slip, this motion could potentially start the engine, which would create an extremely dangerous situation. Second, most hydraulic components are designed to leak, referred to as a case drain, and is caused by bypassing oil that is required to keep the component lubricated. The leakage rate for motors typically employed is approximately 1.1 lit/min at full system pressure and less than one-tenth that at minimum system pressure. When the vehicle is parked, system pressure is typically quite low, however, an external load applied to the motors acts to increase pressure, thereby increasing the leakage rate. Actual experience with hybrid hydraulic transmission equipped vehicles has shown that when parked on a 20 percent grade, the vehicle moves approximately 2 cm/min. While this very slow rate of movement is not perceptible, over extended periods of time, the distance travelled can become large enough to cause potentially dangerous situations to arise.
There are numerous embodiments of transmission brakes for traditional automatic transmission-equipped vehicles known to those skilled in the art. However, since those cannot be applied to hydraulic hybrid transmission-equipped vehicles, a review of prior patents is not provided herein.
3. Four-Wheel Locking in the Present State of the Art:
Four-wheel drive is employed to increase the vehicle's available traction, thereby allowing it to negotiate terrain that might not otherwise be possible. There is a wide variety of four-wheel drive systems available for standard (non-hybrid) vehicles. The first distinction typically recognized is “four-wheel drive” versus “all-wheel drive”. The former usually refers to a part-time system for which the driver specifically commands the change from two-wheel to four-wheel drive. Such systems are almost exclusively engaged for use in low-traction conditions, such as off-road driving. The latter refers to a full-time system that cannot be switched on and off. Such systems are used in a wide range of applications. Within the context of the current disclosure, four-wheel, i.e., part-time, drive systems are the more closely related of the two.
Four-wheel drive systems are comprised of two differentials (one located between the front wheels, the other between the rear wheels) and a transfer case. The differentials send torque to each of the wheels while allowing them to spin at different speeds. The transfer case locks the front and rear drive shafts (also referred to as a prop shaft) together, thereby allowing the engine's torque to be applied to all four wheels.
FIG. 2 shows a simple drawing of a vehicle drive train 200 for a vehicle having a known four-wheel drive system. The four-wheel drive system includes a transfer case 202, a rear drive shaft 204, a rear differential 206, a front drive shaft 208, a front differential 210, a half shaft 212, and a plurality of locking hubs 214 connected to the vehicle's wheels. Each of the components of the vehicle is connected as shown, and thereby forms the vehicle drive train 200.
These four-wheel drive systems, however, do have a weakness. Often, an open differential is employed to evenly divide the torque evenly between each of the two wheels to which it is connected. If one of those two wheels loses traction, the torque applied to that wheel drops to zero. Because the torque is split evenly, this means that the other wheel also receives zero torque. Thus, even if the other wheel has plenty of traction, no torque is transferred to it. A limited slip differential partially alleviates this problem by applying some torque to each wheel regardless of conditions.
Several hydraulic hybrid transmission equipped vehicles provide an all-wheel drive capability. With respect to FIG. 2, these vehicles eliminate the transfer case 202, replace the drives shafts 204, 208 with hydraulic lines, and add hydraulic motors immediately before the differentials 206, 210. In this manner, torque can be applied to all four wheels at all times. However, this vehicle transmission does not address the problem described.
To minimize vehicle weight, simplify component packaging, maximize operating efficiency, and minimize cost, the hydraulic motors on a hydraulic hybrid transmission equipped vehicles are sized to handle only slightly more than half of the total power that can be delivered by the hydraulic system. Considering the situation in which both of the front wheels have limited traction, the hydraulic motor that powers the front wheels will consume some oil (which would spin the front wheels but not provide any motive power) and the hydraulic motor that powers the rear wheels would consume as much oil as possible. The problem is that the amount of power, and thereby torque, being applied to the rear wheels is only half of the total power available.